Amazon CEO: We sell 6 Kindle books to every 10 books

Online retailer Amazon.com had quite a year. Yesterday, the company reported a 42% year-over-year increase in fourth quarter net sales, and a 71% increase in net income. For the full year 2009, Amazon's net sales increased 28% to $24.51 billion, and its net income increased 40% to $902 million.

Amazon tends to keep its Kindle-related sales figures a secret, so there is no concrete proof that the Kindle has meaningfully contributed to Amazon's gains. However, Amazon founder and CEO Jeff Bezos gave an hint at the device's performance in yesterday's earnings call.

"Millions of people now own Kindles, and Kindle owners read...a lot," Bezos remarked. "When we have both editions, we sell 6 Kindle books for every 10 physical books. This is year-to-date and includes only paid books -- free Kindle books would make the number even higher. It's been an exciting 27 months."

Some have predicted that the e-book market still has as long as five years to go before it reaches maturity; but if Amazon's e-book business is already 60% of the size of its physical book business, the Kindle platform could be mature before the rest of the e-book market even gets going.

This is because the Kindle platform includes more than just Amazon's three Kindle-branded e-readers, it also includes the popular iPhone/iPod Touch app and desktop PC Kindle Reader software. With these, users can download Kindle books without having to buy the dedicated hardware.

Even so, Bezos' claim that there are "millions" of Kindle owners is the first statement from Amazon giving even a vague size of the userbase, and it could turn out that Amazon is exceeding expectations. TechCrunch, a site that has long been determined to find out how many Kindles are really out there estimated today that as many as 3 million units have been sold thus far.